Compound semiconductor device and method of manufacturing the same

ABSTRACT

An AlGaN/GaN HEMT includes a compound semiconductor stack structure; an element isolation structure which demarcates an element region on the compound semiconductor stack structure; a first insulating film which is formed on the element region and is not formed on the element isolation structure; a second insulating film which is formed on at least the element isolation structure and is higher in hydrogen content than the first insulating film; and a gate electrode which is formed on the element region of the compound semiconductor stack structure via the second insulating film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2012-077192, filed on Mar. 29,2012, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to a compoundsemiconductor device and a method of manufacturing the same.

BACKGROUND

It has been considered to apply nitride semiconductors tohigh-withstand-voltage, high-power semiconductor devices by utilizingtheir characteristics such as a high saturation electron velocity and awide band gap. For example, GaN being a nitride semiconductor has a bandgap of 3.4 eV, which is higher than a band gap of Si (1.1 eV) and a bandgap of GaAs (1.4 eV), and has high breakdown electric field intensity.This makes GaN very promising as a material of semiconductor devices forpower supply realizing a high voltage operation and a high power.

Many reports have been made on field-effect transistors, in particular,HEMT (High Electron Mobility Transistor) as devices using nitridesemiconductors. For example, among GaN-based HEMT (GaN-HEMT), anAlGaN/GaN HEMT using GaN as an electron transit layer and using AlGaN asan electron supply layer has been drawing attention. In the AlGaN/GaNHEMT, a distortion ascribable to a difference in lattice constantbetween GaN and AlGaN occurs in AlGaN. Owing to piezoelectricpolarization caused by the distortion and spontaneous polarization ofAlGaN, high-concentration two-dimensional electron gas (2DEG) isobtained. Therefore, the AlGaN/GaN HEMT is expected as a high-efficiencyswitch element or a high withstand voltage, high power device forelectric vehicles and the like.

[Patent Document 1] Japanese Laid-open Patent Publication No.2010-219247

In a nitride semiconductor device, a protection film is often formedthrough the deposition of an insulator covering a nitride semiconductorlayer. In some case, what is called a MIS-type HEMT is formed with thisprotection film used as a gate insulating film. In the case where theprotection film is formed, high-temperature annealing is applied to theprotection film after its formation to improve its insulating filmquality.

However, there has been found a problem that the high-temperatureannealing, though improving the insulating film quality of theprotection film, increases an off-leakage current in the nitridesemiconductor device.

FIG. 1 is a characteristic chart illustrating a correlation of theoff-leakage current with drain voltage in an AlGaN/GaN HEMT having aprotection film. The protection film was formed by an ALD method (AtomicLayer Deposition method) with aluminum oxide used as a material. When aprocessing temperature is low (for example, 600° C.), there is almost noproblem of the off-leakage current. On the other hand, when theprocessing temperature is high temperature (for example, 720° C.) atwhich the insulating film quality of the protection film significantlyimproves, it has been found out that the off-leakage current increasesin accordance with an increase in the drain voltage.

SUMMARY

A compound semiconductor device according to an aspect includes: acompound semiconductor region; an element isolation structure whichdemarcates an element region on the compound semiconductor region; afirst insulating film which is formed on the element region and is notformed on the element isolation structure; and a second insulating filmwhich is formed on at least the element isolation structure and ishigher in hydrogen content than the first insulating film.

A method of manufacturing a semiconductor device according to an aspectincludes: forming, on a compound semiconductor region, a firstinsulating film which has an opening on an element isolation region andcovers an element region; forming an element isolation structure on theelement isolation region; and forming a second insulating film whichcovers at least the element isolation structure and is higher inhydrogen content than the first insulating film.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a characteristic chart illustrating a correlation of anoff-leakage current with drain voltage in an AlGaN/GaN HEMT having aprotection film.

FIG. 2 is a schematic cross-sectional view illustrating a method ofmanufacturing a MIS-type AlGaN/GaN HEMT according to a first embodimentin order of steps.

FIG. 3, which is continued from FIG. 2, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 4, which is continued from FIG. 3, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 5, which is continued from FIG. 4, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 6, which is continued from FIG. 5, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 7, which is continued from FIG. 6, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 8, which is continued from FIG. 7, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 9, which is continued from FIG. 8, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 10, which is continued from FIG. 9, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 11, which is continued from FIG. 10, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 12, which is continued from FIG. 11, is a schematic cross-sectionalview illustrating the method of manufacturing the MIS-type AlGaN/GaNHEMT according to the first embodiment in order of steps.

FIG. 13 is a schematic cross-sectional view illustrating an AlGaN/GaNHEMT of a comparative example.

FIG. 14 is a characteristic chart illustrating a result of a study on acorrelation of off-leakage current with PDA temperature in thecomparative example.

FIG. 15 is a characteristic chart illustrating a result of a study on acorrelation of a 2DEG sheet resistance value with the PDA temperature.

FIG. 16 is a characteristic chart illustrating a result of a study on acorrelation of moisture concentration in a protection film withannealing temperature.

FIG. 17 is a characteristic chart illustrating a result obtained when acorrelation of the off-leakage current with drain voltage in theAlGaN/GaN HEMT according to the embodiment is studied based on thecomparison with the comparative example.

FIG. 18 is a schematic cross-sectional view illustrating a main step ofa method of manufacturing a MIS-type AlGaN/GaN HEMT according to amodification example of the first embodiment.

FIG. 19, which is continued from FIG. 18, is a schematic cross-sectionalview illustrating a main step of the method of manufacturing theMIS-type AlGaN/GaN HEMT according to the modification example of thefirst embodiment.

FIG. 20 is a schematic cross-sectional view illustrating a main step ofa method of manufacturing a Schottky-type AlGaN/GaN HEMT according to asecond embodiment.

FIG. 21, which is continued from FIG. 20, is a schematic cross-sectionalview illustrating a main step of the method of manufacturing theSchottky-type AlGaN/GaN HEMT according to the second embodiment.

FIG. 22 is a schematic cross-sectional view illustrating a main step ofa method of manufacturing a Schottky-type AlGaN/GaN HEMT according to amodification example of the second embodiment.

FIG. 23, which is continued from FIG. 22, is a schematic cross-sectionalview illustrating a main step of the method of manufacturing theSchottky-type AlGaN/GaN HEMT according to the modification example ofthe second embodiment.

FIG. 24 is a schematic plane view illustrating a HEMT chip which usesthe AlGaN/GaN HEMT of one kind selected from the first and secondembodiments and the modification examples thereof.

FIG. 25 is a schematic plane view illustrating a discrete package of aHEMT chip which uses the AlGaN/GaN HEMT of one kind selected from thefirst and second embodiments and the modification examples thereof.

FIG. 26 is a connection diagram illustrating a PFC circuit according toa third embodiment.

FIG. 27 is a connection diagram illustrating a schematic structure of apower supply device according to a fourth embodiment.

FIG. 28 is a connection diagram illustrating a schematic structure of ahigh-frequency amplifier according to a fifth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be described in detail with reference tothe drawings. In the following embodiments, structures of compoundsemiconductor devices will be described together with methods ofmanufacturing the same.

Note that in the following drawings, some constituent members are notillustrated with accurate relative size and thickness for convenience ofthe illustration.

First Embodiment

In this embodiment, a MIS-type AlGaN/GaN HEMT will be disclosed as thecompound semiconductor device.

FIG. 2 to FIG. 12 are schematic cross-sectional views illustrating amethod of manufacturing the MIS-type AlGaN/GaN HEMT according to thefirst embodiment in order of steps.

First, as illustrated in FIG. 2, a compound semiconductor region, here,a compound semiconductor stack structure 2 is formed on, for example, aSi substrate 1 as a growth substrate. Instead of the Si substrate, a SiCsubstrate, a sapphire substrate, a GaAs substrate, a GaN substrate, orthe like may be used as the growth substrate. Further, conductivity ofthe substrate may be either semi-insulative or conductive.

The compound semiconductor stack structure 2 includes a nucleusformation layer 2 a, an electron transit layer 2 b, an intermediatelayer (spacer layer) 2 c, an electron supply layer 2 d, and a cap layer2 e. The cap layer 2 e has a three-layer structure and is composed of afirst cap 2 e 1, a second cap 2 e 2, and a third cap 2 e 3 which arestacked in sequence.

In more detail, the following compound semiconductors are grown on theSi substrate 1 by, for example, a MOVPE (Metal Organic Vapor PhaseEpitaxy) method. Instead of the MOVPE method, a MBE (Molecular BeamEpitaxy) method or the like may be used.

On the Si substrate 1, the compound semiconductors that are to be thenucleus formation layer 2 a, the electron transit layer 2 b, theintermediate layer 2 c, the electron supply layer 2 d, and the cap layer2 e are grown in sequence. The nucleus formation layer 2 a is formed ofAlN grown on the Si substrate 1 with a thickness of, for example, about0.1 μm. The electron transit layer 2 b is formed of i (intentionallyundoped)-GaN grown with a thickness of, for example, 3 μm. Theintermediate layer 2 c is formed of i-AlGaN grown with a thickness of,for example, about 5 nm. The electron supply layer 2 d is formed ofn-AlGaN grown with a thickness of about 30 nm. In forming the cap layer2 e, n-GaN is grown with, for example, 7 nm as the first cap 2 e 1, AlNis grown with, for example, about 2 nm as the second cap 2 e 2, andn-GaN is grown with, for example, about 4 nm as the third cap 2 e 3. Theintermediate layer 2 c is not sometimes formed. The electron supplylayer 2 d may be formed of i-AlGaN.

To grow GaN, mixed gas of trimethylgallium (TMGa) gas being a Ga sourceand ammonia (NH₃) gas is used as a source gas. To grow AlGaN, mixed gasof trimethylaluminum (TMAl) gas, TMGa gas, and NH₃ gas is used as asource gas. According to the compound semiconductor layer to be grown,whether or not to supply the TMAl gas and the TMGa gas and flow ratesare appropriately set. The flow rate of the NH₃ gas being a commonsource is set to about 100 sccm to about 10 slm. Further, growthpressure is set to about 50 Torr to about 300 Torr, and growthtemperature is set to about 1000° C. to about 1200° C.

In order to grow AlGaN and GaN as an n-type, that is, in order to formthe electron supply layer 2 d (n-AlGaN) and the first and third caps 2 e1, 2 e 3 (n-GaN), n-type impurities are added to the source gases ofAlGaN and GaN. Here, for example, silane (SiH₄) gas containing Si isadded to the source gases at a predetermined flow rate, thereby dopingAlGaN and GaN with Si. A doping concentration of Si is set to about1×10¹⁸/cm³ to about 1×10²⁰/cm³, for example, set to about 5×10¹⁸/cm³.

In the formed compound semiconductor stack structure 2, piezoelectricpolarization caused by a distortion ascribable to a difference between alattice constant of GaN and a lattice constant of AlGaN occurs on aninterface of the electron transit layer 2 b with the electron supplylayer 2 d (to be precise, an interface with the intermediate layer 2 c.hereinafter, referred to as a GaN/AlGaN interface). Owing to both aneffect of the piezoelectric polarization and an effect of spontaneouspolarization of the electron transit layer 2 b and the electron supplylayer 2 d, two-dimensional electron gas (2DEG) with high electronconcentration is generated in the GaN/AlGaN interface.

Subsequently, as illustrated in FIG. 3, recesses 2A are formed inportions where to form gate electrodes.

In more detail, first, a resist is applied on a surface of the compoundsemiconductor stack structure 2. The resist is processed by lithography,whereby openings from which portions corresponding to the portions whereto form the gate electrodes, of the surface of the compoundsemiconductor stack structure are exposed are formed in the resist.Consequently, a resist mask having the openings is formed.

By using this resist mask, the compound semiconductor stack structure 2is dry-etched until a surface layer of the electron transit layer 2 b isetched, here, up to an about depth where 2DEG generated in the interfaceof the electron transit layer 2 b is split. Consequently, the recesses2A from whose bottom surfaces, parts of the etched electron transitlayer 2 b are exposed are formed in the compound semiconductor stackstructure 2. Thus forming the recesses 2A enables what is called anormally-off operation. For the dry etching, inert gas such as Ar andchlorine-based gas such as Cl₂ is used as etching gas.

The resist mask is removed by wet processing, asking, or the like.

Subsequently, as illustrated in FIG. 4, an Al₂O₃ film 3A is formed.

In more detail, for example, aluminum oxide (Al₂O₃) is deposited on thewhole surface of the compound semiconductor stack structure 2 so as tofill the recesses 2A. Al₂O₃ is deposited with a film thickness of about40 nm at a processing temperature of about 300° C. by, for example, theALD method. Instead of Al₂O₃, hafnium oxide (HfO₂), aluminum oxynitride(AlON), or tantalum oxide (Ta₂O₅) or any combination thereof may bedeposited.

Consequently, the Al₂O₃ film 3A covering the whole surface of thecompound semiconductor stack structure 2 is formed.

Subsequently, as illustrated in FIG. 5, high-temperature annealing isapplied to the Al₂O₃ film 3A to form a first insulating film 3.

In more detail, the Al₂O₃ film 3A is subjected to the one-minutehigh-temperature annealing at a processing temperature of 700° C. orhigher, here, 850° C. which is higher than that of later-describedlow-temperature annealing. By this high-temperature annealing, the Al₂O₃film 3A is reformed into a film having excellent insulating filmquality, with its hydrogen content being lower than a later-describedsecond insulating film. The Al₂O₃ film 3A having undergone thehigh-temperature annealing is the first insulating film 3. The hydrogencontent of the first insulating film 3 becomes 1% or less, here about0.5%. The “hydrogen content” means a ratio of an amount of hydrogenatoms to an amount of Al atoms per unit volume (1 cm³). The hydrogenconcentration of the first insulating film 3, as evaluated by a thermaldesorption spectrometry method (TDS method), is about 5×10¹⁹/cm³ orless, here about 1×10¹⁹/cm³.

Subsequently, as illustrated in FIG. 6, openings 3 a are formed in thefirst insulating film 3.

In more detail, a resist is first applied on the first insulating film 3and is processed by lithography. Consequently, a resist mask 10 havingopenings 10 a from which element isolation regions (portions where toform element isolation structures) of the first insulating film 3 areexposed is formed.

By using the resist mask 10, the first insulating film 3 is dry-etched.As etching gas, SF₆ is used, for instance. Consequently, portions on theelement isolation regions, of the first insulating film 3 are removed,whereby the openings 3 a from which the element isolation regions areexposed are formed in the first insulating film 3.

Subsequently, as illustrated in FIG. 7, element isolation structures 4are formed.

In more detail, by using the resist mask 10 again, argon (Ar), forexample, is injected to the element isolation regions of the compoundsemiconductor stack structure 2. An injection condition is such thatacceleration energy of Ar is about 40 keV and its dose amount is about1×10¹⁴/cm². Consequently, the element isolation structures 4 are formedin the compound semiconductor stack structure 2 and a surface layerportion of the Si substrate 1. The element isolation structures 4demarcate element regions on the compound semiconductor stack structure2.

Incidentally, for the element isolation, instead of the aforesaidinjection method, another known method such as, for example, a STI(Shallow Trench Isolation) method may be used. At this time, for the dryetching of the compound semiconductor stack structure 2, achlorine-based etching gas is used, for instance.

The resist mask 10 is removed by wet processing, asking, or the like.

Subsequently, as illustrated in FIG. 8, an Al₂O₃ film 5A is formed.

In more detail, oxide aluminum (Al₂O₃), for example, is deposited on thewhole surface of the compound semiconductor stack structure 2 includingareas on the element isolation structures 4. Al₂O₃ is deposited with anabout 20 nm film thickness at a processing temperature of about 300° C.,by, for example, the ALD method. Instead of Al₂O₃, hafnium oxide (HfO₂),aluminum oxynitride (AlON), or tantalum oxide (Ta₂O₅) or any combinationthereof may be deposited by, for example, the ALD method.

Consequently, the Al₂O₃ film 5A covering the whole surface of thecompound semiconductor stack structure 2 including the areas on theelement isolation structures 4 is formed.

Subsequently, as illustrated in FIG. 9, the low-temperature annealing isapplied to the Al₂O₃ film 5A to form the second insulating film 5.

In more detail, the Al₂O₃ film 5A is subjected to the one-minutelow-temperature annealing at a processing temperature of 700° C. orlower, here 600° C. which is lower than that of the aforesaidhigh-temperature annealing. By this low-temperature annealing, the Al₂O₃film 5A becomes Al₂O₃ higher in hydrogen content than the aforesaidfirst insulating film 3. The Al₂O₃ film 5A having undergone thelow-temperature annealing is the second insulating film 5. The hydrogencontent of the second insulating film 5 becomes 1% or more, here about10%, which is higher than that of the first insulating film 3. Thehydrogen concentration of the second insulating film 5, as evaluated bythe thermal desorption spectrometry method (TDS method), is 5×10¹⁹/cm³or more, here about 5×10¹⁹/cm³.

Subsequently, as illustrated in FIG. 10, source electrodes 6 and drainelectrodes 7 are formed.

In more detail, a resist is first applied on the surface of the compoundsemiconductor stack structure 2. The resist is processed by lithography,whereby openings from which portions corresponding to portions where toform the source electrodes and the drain electrodes, of the surface ofthe compound semiconductor layer structure 2 are exposed are formed.Consequently, a resist mask having the openings is formed.

By using this resist mask, the cap layer 2 e is dry-etched until asurface of the electron supply layer 2 d is exposed. Consequently,recesses 2B, 2C for electrodes from which the portions where to form thesource electrodes and the drain electrodes on the surface of theelectron supply layer 2 d are exposed are formed in the cap layer 2 e.For the dry etching, inert gas such as Ar and chlorine-based gas such asCl₂ are used as etching gas. Incidentally, to form the recesses 2B, 2Cfor electrodes, the etching may be performed up to a middle of the caplayer 2 e or the etching may be performed up to a predetermined depthfrom the electron supply layer 2 d.

The resist mask is removed by wet processing, asking, or the like.

Next, a resist mask for forming the source electrodes and the drainelectrodes is formed. Here, an eaves-structure double-layer resistsuitable for a vapor deposition method and a liftoff method is used, forinstance. This resist is applied on the compound semiconductor stackstructure 2, and openings from which the recesses 2B, 2C for electrodesare exposed are formed. Consequently, the resist mask having theopenings is formed.

By using this resist mask, Ta/Al, for example, are deposited aselectrode materials on the resist mask including the inside of therecesses 2B, 2C for electrodes by, for example, the vapor depositionmethod. A thickness of Ta is about 30 nm and a thickness of Al is about200 nm. The resist mask and Ta/Al deposited thereon are removed by theliftoff method. Thereafter, the Si substrate 1 is heat-treated, forexample, in a nitride atmosphere at a temperature of about 400° C. toabout 1000° C., for example, about 600° C., and the residual Ta/Al arebrought into ohmic contact with the electron supply layer 2 d. If theohmic contact of Ta/Al and the electron supply layer 2 d is obtained,the heat treatment is not sometimes necessary. Through the aboveprocesses, the source electrodes 6 and the drain electrodes 7 formed ofparts of the electrode materials filling the recesses 2B, 2C forelectrodes of the cap layer 2 e are formed.

Subsequently, as illustrated in FIG. 11, recesses 8 for electrodes areformed in portions where to form gate electrodes.

In more detail, a resist is first applied on the whole surface includingareas on the second insulating film 5. The resist is processed bylithography, whereby openings from which portions corresponding to theportions where to form the gate electrodes, of the surface of the secondinsulating film 5 are exposed are formed in the resist. Consequently, aresist mask having the opening is formed.

By using this resist mask, the second insulating film 5 and the firstinsulating film 3 in the recesses 2A are dry-etched so that the firstinsulating film 3 remains on bottom portions with a predeterminedthickness. Consequently, in the first insulating film 3 and the secondinsulating film 5 in the recesses 2A, the recesses 8 for electrodes onwhose bottom portions the first insulating film 3 remains with thepredetermined thickness are formed. The first insulating film 3 on thebottom portions function as a gate insulating film. For the dry etching,SF₆ is used as etching gas.

The resist mask is removed by wet processing, ashing, or the like.

Subsequently, the gate electrodes 9 are formed as illustrated in FIG.12.

In more detail, a resist mask for forming the gate electrodes is firstformed. Here, an eaves-structure double-layer resist suitable for thevapor deposition method and the liftoff method is used, for instance.This resist is applied on the whole surface, and openings from which therecesses 8 for electrodes are exposed are formed. Consequently, theresist mask having the opening is formed.

By using this resist mask, Ni/Au, for example, are deposited aselectrode materials on the resist mask including the inside of theopenings from which the recesses 8 for electrodes are exposed, by, forexample, the vapor deposition method. A thickness of Ni is about 30 nmand a thickness of Au is about 400 nm. The resist mask and Ni/Audeposited thereon are removed by the liftoff method. Through the aboveprocesses, the gate electrodes 9 filling the inside of the recesses 8for electrodes and projecting above the second insulating film 5 areformed. The first insulating film 3 under the gate electrodes 9 becomesa gate insulating film.

Thereafter, through steps such as the formation of wirings connected tothe source electrodes 6, the drain electrodes 7, and the gate electrodes9, the MIS-type AlGaN/GaN HEMT according to this embodiment is formed.

Here, operations and effects that the AlGaN/GaN HEMT according to thisembodiment has will be described based on the comparison with acomparative example.

FIG. 13 is a schematic cross-sectional view illustrating an AlGaN/GaNHEMT of the comparative example. In FIG. 13, the same constituentmembers and so on as those of the AlGaN/GaN HEMT according to thisembodiment are denoted by the same reference signs.

In the AlGaN/GaN HEMT of the comparative example, a protection film 15is formed instead of forming the first insulating film 3 and the secondinsulating film 5 in this embodiment. The other structures are the sameas those of this embodiment, and therefore in FIG. 13, they are denotedby the same reference signs as those in FIG. 12. The protection film 15is formed on the whole surface of a compound semiconductor stackstructure 2 (including areas on element isolation structures 4) in orderto protect an element surface.

In the comparative example where the protection film 15 is formed, inthe element isolation structure 4, an off-leakage current flows in theelement isolation structure 4 between adjacent drain electrode 7 andsource electrode 8 in the AlGaN/GaN HEMT as illustrated by the arrow Ain FIG. 13.

In this embodiment, attention is focused on a correlation of theoff-leakage current with the protection film 15. FIG. 14 is acharacteristic chart illustrating a result of a study on a correlationof the off-leakage current with PDA temperature in the comparativeexample. The PDA (Post Deposition Anneal) temperature representstemperature of annealing after the protection film is formed. In FIG.14, four kinds of samples were fabricated for the AlGaN/GaN HEMT of thecomparative example having the protection film 15. These samples areformed in such a manner that, after Al₂O₃ films are formed by the ALDmethod, they are annealed for one minute at processing temperatures of600° C., 700° C., 720° C., and 750° C. respectively. The samples will becalled samples 1 to 4 respectively.

As is seen in FIG. 14, in the sample 1, the off-leakage currentpresented an insignificantly low value. On the other hand, in thesamples 2, 3, 4, the off-leakage current presented large values. It hasbecome clear that the off-leakage current thus has an obviouscorrelation with the PDA temperature.

Keeping the result in FIG. 14 in mind, a correlation of an amount of2DEG generated in the AlGaN/GaN HEMT of the comparative example with thePDA temperature was studied. The amount of the 2DEG generated is largeras a sheet resistance value is lower.

FIG. 15 is a characteristic chart illustrating a result of a study on acorrelation of the 2DEG sheet resistance value with the PDA temperature.The broken line in FIG. 15 represents the 2DEG sheet resistance value ata stage when the compound semiconductor stack structure 2 is epitaxiallygrown. In FIG. 15, four kinds of samples were fabricated for theAlGaN/GaN HEMT of the comparative example having the protection film 15.These samples are formed in such a manner that, after Al₂O₃ films areformed by the ALD method, they are annealed for one minute at processingtemperatures of 600° C., 700° C., 750° C., 800° C. respectively. Thesamples will be called samples 1 to 4 respectively.

As is seen in FIG. 15, in the sample 1, the 2DEG sheet resistance valuepresents a value close to that at the time of the growth of the compoundsemiconductor stack structure 2, which indicates that the 2DEG amount isclose to a desired value. On the other hand, in the samples 2, 3, 4, the2DEG sheet resistance value is low, which indicates that the 2DEG amountis larger than the desired value. A possible reason why the 2DEG amountthus increases when the protection film is annealed at the processingtemperature of 700° C. or higher is that the high-temperature annealingdecreases an energy band on surfaces of the element isolationstructures.

Based on the result in FIG. 15, it was inferred that the decrease of theenergy band on the surfaces of the element isolation structures wasascribable to a change in the hydrogen content (moisture content) in theprotection film caused by the annealing, and a correlation of themoisture concentration in the protection film with the annealingtemperature was studied. A result thereof is illustrated in FIG. 16. Asillustrated in FIG. 16, as compared with the moisture content of theprotection film not having undergone the annealing, the moisture contentreduces as the annealing temperature increases, and when the annealingtemperature is set to 700° C. and 800° C., the moisture in theprotection film is almost completely removed.

In this embodiment, on the element regions, the first insulation film 3having high insulating film quality, that is, having undergone thehigh-temperature annealing is formed as the protection film as describedabove. On the other hand, on the element isolation structures 4, thefirst insulating film 3 is not formed, and instead, the secondinsulating film 5 higher in hydrogen content than the first insulatingfilm 2, that is, having undergone the low-temperature annealing isformed.

In the AlGaN/GaN HEMT according to this embodiment, a correlation of theoff-leakage current with drain voltage was studied based on thecomparison with a comparative example. The measurement result isillustrated in FIG. 17. In FIG. 17, the comparative example is theAlGaN/GaN HEMT in which the protection film 15 is formed by the 700° C.,high-temperature annealing in FIG. 13. As is seen in FIG. 17, in thecomparative example, the off-leakage current presented high values overthe whole measurement range from 0 V to 400 V of the drain voltage, andit increased as the drain voltage increased. On the other hand, in thisembodiment, the off-leakage current presented almost no change andpresented low values over the whole measurement range from 0 V to 400 Vof the drain voltage. It has been found out that, in this embodiment,since the first insulating film 3 being the protection film of theelement regions does not contain hydrogen, a stable transistor operationis exhibited, but surface leakage paths are not formed on the elementisolation structures 4, and the off-leakage current is greatly improved.In the AlGaN/GaN HEMT according to this embodiment, owing to theimprovement in the off-leakage current, transistor reliability alsoimproves, and an average life of 1×10⁶ hours was confirmed under 200°C., high-temperature electricity supply when the drain voltage was 400V.

As described above, this embodiment realizes a highly reliable MIS-typeAlGaN/GaN HEMT in which the protection film (first insulating film 3)functioning also as the gate insulating film is formed to have excellentinsulating film quality but the occurrence of the off-leakage current issurely inhibited, enabling a reduction in a loss at the power-off time.

In the foregoing, the example where Al₂O₃ is deposited as the first andsecond insulating films is presented. When HfO₂ is formed instead ofAl₂O₃ as the first and second insulating films, they are formed in thefollowing manner, for instance. A HfO₂ film is formed by the atomiclayer deposition (ALD) method or the like, this HfO₂ film is subjectedto 700° C. high-temperature annealing for one minute, whereby the firstinsulating film is formed. Similarly, a HfO₂ film is formed by the ALDmethod or the like, and this HfO₂ film is subjected to 500° C.low-temperature annealing for one minute, whereby the second insulatingfilm is formed.

When AlON is formed instead of Al₂O₃ as the first and second insulatingfilms, they are formed in the following manner, for instance. An AlONfilm is formed by the ALD method or the like, and this AlON film issubjected to 750° C. high-temperature annealing for one minute, wherebythe first insulating film is formed. Similarly, an AlON film is formedby the ALD method or the like and this AlON film is subjected to 600° C.low-temperature annealing for one minute, whereby the second insulatingfilm is formed. Similarly to the AlGaN/GaN HEMT having the first andsecond insulating films made of Al₂O₃, an AlGaN/GaN HEMT having thefirst and second insulating films made of AlON presented a lowoff-leakage current, but could realize a high on-current (about 1.5times as that of the comparative example in FIG. 13). This is becausethe first and second insulating films made of AlON are formed as filmshaving little trap and deep levels trapping electrons are decreased.Thus, a secondary effect of suppressing an increase in on-resistance hasalso been confirmed.

When Ta₂O₅ is formed instead of Al₂O₃ as the first and second insulatingfilms, they are formed in the following manner, for instance. A Ta₂O₅film is formed by a sputtering method or the like and this Ta₂O₅ film issubjected to 600° C. high-temperature annealing for one minute, wherebythe first insulating film is formed. Similarly, a Ta₂O₅ film is formedby the sputtering method or the like and this Ta₂O₅ film is subjected to300° C. low-temperature annealing for one minute, whereby the secondinsulating film is formed.

Modification Example

Here, a modification example of this embodiment will be described. Inthis embodiment, the second insulating film 5 on the element regions isleft, taking a reduction in steps of the manufacturing process intoconsideration, but the second insulating film 5 on the element regionsmay be removed.

FIG. 18 and FIG. 19 are schematic cross-sectional views illustratingmain steps of a manufacturing method of a MIS-type AlGaN/GaN HEMTaccording to the modification example of the first embodiment. The sameconstituent members and so on as those of the first embodiment will bedenoted by the same reference signs and a detailed description thereofwill be omitted.

In this modification example, the steps in FIG. 2 to FIG. 9 are firstperformed as in the first embodiment.

Subsequently, as illustrated in FIG. 18, a second insulating film 5 onelement regions is removed.

In more detail, a resist mask covering only portions on elementisolation structures 4, of the second insulating film 5 is formed bylithography. By using this resist mask, the second insulating film 5 iswet-etched using a predetermined etching solution. Consequently, thesecond insulating film 5 on the element regions is removed and thesecond insulating film 5 is left only on the element isolationstructures 4.

The resist mask is removed by wet processing, ashing, or the like.

Thereafter, as illustrated in FIG. 19, the steps in FIG. 10 to FIG. 12are performed to form source electrodes 6, drain electrodes 7, and gateelectrodes 9, as in the first embodiment.

Thereafter, through steps such as the formation of wirings connected tothe source electrodes 6, the drain electrodes 7, and the gate electrodes9, the MIS-type AlGaN/GaN HEMT according to this modification example isformed.

This modification example realizes a highly reliable MIS-type AlGaN/GaNHEMT in which a protection film (first insulating film 3) functioningalso as a gate insulating film is formed to have excellent insulatingfilm quality but the occurrence of an off-leakage current is surelyinhibited, enabling a reduction in a loss at the power-off time.

Second Embodiment

In this embodiment, a Schottky-type AlGaN/GaN HEMT is disclosed as thecompound semiconductor device.

FIG. 20 and FIG. 21 are schematic cross-sectional views illustratingmain steps of a manufacturing method of the Schottky-type AlGaN/GaN HEMTaccording to the second embodiment. The same constituent members and soon as those of the first embodiment will be denoted by the samereference signs and a detailed description thereof will be omitted.

In this embodiment, the steps in FIG. 2 to FIG. 10 are first performedas in the first embodiment.

Subsequently, as illustrated in FIG. 20, recesses 11 for electrodes areformed at portions where to form gate electrodes.

In more detail, a resist is applied on the whole surface including areason a second insulating film 5. The resist is processed by lithography,whereby openings from which portions corresponding to the portions whereto form the gate electrodes, of a surface of the second insulating film5 are exposed are formed in the resist. Consequently, a resist maskhaving the openings is formed.

By using this resist mask, the second insulating film 5 and a firstinsulating film 3 in recesses 2A are dry-etched until an electrontransit layer 2 b on bottom surfaces of the recesses 2A are exposed.Consequently, in the first insulating film 3 and the second insulatingfilm 5 in the recesses 2A, the recesses 11 for electrodes from whosebottom portions the electron transit layer 2 b is exposed are formed.For the dry etching, Cl₂ is used as etching gas.

The resist mask is removed by wet processing, ashing, or the like.

Subsequently, as illustrated in FIG. 21, gate electrodes 12 are formed.

In more detail, a resist mask for forming the gate electrodes is firstformed. Here, an eaves-structure double-layer resist suitable for avapor deposition method and a lift-off method is used, for instance.This resist is applied on the whole surface, and openings from which therecesses 11 for electrodes are exposed are formed. Consequently, theresist mask having the openings is formed.

By using this resist mask, for example, Ni/Au are deposited as electrodematerials by, for example, the vapor deposition method on the resistmask including the inside of the openings from which the recesses 11 forelectrodes are exposed. A thickness of Ni is about 30 nm and a thicknessof Au is about 400 nm. The resist mask and Ni/Au deposited thereon areremoved by the lift-off method. Consequently, the gate electrodes 12filling the inside of the recesses 11 for electrodes and projectingabove the second insulating film 5 are formed. The gate electrodes 12are in Schottky contact with the electron transit layer 2 b.

Thereafter, through steps such as the formation of wirings connected tosource electrodes 6, drain electrodes 7, and the gate electrodes 12, theSchottky-type AlGaN/GaN HEMT according to this embodiment is formed.

As described above, this embodiment realizes a highly reliableSchottky-type AlGaN/GaN HEMT in which a first insulating film 3 being aprotection film of a compound semiconductor stack structure 2 is formedto have excellent insulating film quality but the occurrence of anoff-leakage current is surely inhibited, enabling a reduction in a lossat the power-off time.

Modification Example

Here, a modification example of this embodiment will be described. Thisembodiment adopts the structure in which the recesses 2A for electrodesare formed prior to the formation of the recesses 11 for electrodes, andthe first insulating film 3 serving as the protection film of thecompound semiconductor stack structures 2 is filled therein, but therecesses 2A for electrodes do not necessarily have to be formed.

FIG. 22 and FIG. 23 are schematic cross-sectional views illustratingmain steps of a manufacturing method of a Schottky-type AlGaN/GaN HEMTaccording to the modification example of the second embodiment. The sameconstituent members and so on as those of the first and secondembodiments will be denoted by the same reference signs and a detaileddescription thereof will be omitted.

In this modification example, after the step in FIG. 2, the step in FIG.3 is not performed as is performed in the first embodiment, and thesteps in FIG. 4 to FIG. 10 are first performed.

Subsequently, as illustrated in FIG. 22, recesses 13 for electrodes areformed in portions where to form gate electrodes.

In more detail, a resist is first applied on the whole surface includingareas on a second insulating film 5. The resist is processed bylithography, whereby openings from which portions corresponding to theportions where to form the gate electrodes, of a surface of a secondinsulating film 5 are exposed are formed in the resist. Consequently, aresist mask having the openings is formed.

By using this resist mask, the second insulating film 5 and a firstinsulating film 3 are dry-etched until a surface of a compoundsemiconductor stack structure 2 (surface of a cap layer 2 e) is exposed.Consequently, recesses 13 for electrodes from whose bottom portions thesurface of the cap layer 2 e is exposed are formed in the firstinsulating film 3 and the second insulating film 5. For the dry etching,SF, is used as etching gas.

The resist mask is removed by wet processing, ashing, or the like.

Subsequently, gate electrodes 14 are formed as illustrated in FIG. 23.

In more detail, a resist mask for forming the gate electrodes is firstformed. Here, an eaves-structure double-layer resist suitable for avapor deposition method and a lift-off method is used, for instance.This resist is applied on the whole surface, and openings from which therecesses 13 for electrodes are exposed are formed. Consequently, theresist mask having the openings is formed.

By using this resist mask, for example, Ni/Au are deposited as electrodematerials by, for example, the vapor deposition method on the resistmask including the inside of the openings from which the recesses 11 forelectrodes are exposed. A thickness of Ni is about 30 nm and a thicknessof Au is about 400 nm. The resist mask and Ni/Au deposited thereon areremoved by the lift-off method. Through these processes, the gateelectrodes 14 filling the inside of the recesses 13 for electrodes andprojecting above the second insulating film 5 are formed. The gateelectrodes 14 are in Schottky contact with the cap layer 2 e.

Thereafter, through steps such as the formation of wirings connected tosource electrodes 6, drain electrodes 7, and the gate electrodes 14, theSchottky-type AlGaN/GaN HEMT according to this modification example isformed.

As described above, this modification example realizes a highly reliableSchottky-type AlGaN/GaN HEMT in which the first insulating film 3 beinga protection film of a compound semiconductor stack structure 2 isformed to have excellent insulating film quality but the occurrence ofan off-leakage current is surely inhibited, enabling a reduction in aloss at the power-off time.

The AlGaN/GaN HEMT of one kind selected from the first and secondembodiments and the modification examples thereof is applied to what iscalled a discrete package.

In this discrete package, a chip of the AlGaN/GaN HEMT of one kindselected from the first and second embodiments and the modificationexamples thereof is mounted. Hereinafter, the discrete package of thechip of the AlGaN/GaN HEMT of one kind selected from the first andsecond embodiments and the modification examples thereof (hereinafter,referred to as a HEMT chip) will be exemplified.

A schematic structure of the HEMT chip will be illustrated in FIG. 24.

On a surface of the HEMT chip 100, there are provided a transistorregion 101 of the above-described AlGaN/GaN HEMT, a drain pad 102 towhich the drain electrode is connected, a gate pad 103 to which the gateelectrode is connected, and a source pad 104 to which the sourceelectrode is connected.

FIG. 25 is a schematic plane view illustrating the discrete package.

To fabricate the discrete package, the HEMT chip 100 is first fixed to alead frame 112 by using a die attach agent 111 such as solder. A drainlead 112 a is integrally formed with the lead frame 112, and a gate lead112 b and a source lead 112 c are arranged apart from each other asseparate structures from the lead frame 112.

Subsequently, by bonding using Al wires 113, the drain pad 102 and thedrain lead 112 a, the gate pad 103 and the gate lead 112 b, and thesource pad 104 and the source lead 112 c are electrically connected.

Thereafter, by using mold resin 114, the HEMT chip 100 is resin-sealedby a transfer mold method and the lead frame 112 is separated. Throughthe above processes, the discrete package is formed.

Third Embodiment

In this embodiment, a PFC (Power Factor Correction) circuit includingthe AlGaN/GaN HEMT of one kind selected from the first and secondembodiments and the modification examples thereof will be disclosed.

FIG. 26 is a connection diagram illustrating the PFC circuit.

The PFC circuit 20 includes a switch element (transistor) 21, a diode22, a choke coil 23, capacitors 24, 25, a diode bridge 26, and an ACpower source (AC) 27. The AlGaN/GaN HEMT of one kind selected from thefirst and second embodiments and the modification examples thereof isapplied as the switch element 21.

In the PFC circuit 20, a drain electrode of the switch element 21 isconnected to an anode terminal of the diode 22 and one terminal of thechoke coil 23. A source electrode of the switch element 21 is connectedto one terminal of the capacitor 24 and one terminal of the capacitor25. The other terminal of the capacitor 24 and the other terminal of thechoke coil 23 are connected to each other. The other terminal of thecapacitor 25 and a cathode terminal of the diode 22 are connected toeach other. Between the both terminals of the capacitor 24, the AC 27 isconnected via the diode bridge 26. Between the both terminals of thecapacitor 25, a DC power source (DC) is connected. Incidentally, anot-shown PFC controller is connected to the switch element 21.

Regarding the PFC circuit 30, its operating efficiency was studied basedon the comparison with a PFC circuit including the AlGaN/GaN HEMT of thecomparative example illustrated in FIG. 13. The PFC circuit of thecomparative example and the PFC circuit 30 are operated under a 200 Vinput voltage, a 48 V output voltage, and 100 kHz. As a result, theefficiency was about 95% in the PFC circuit of the comparative example.On the other hand, in the PFC circuit 30, the efficiency was about97.5%, and it has been confirmed that a loss is reduced to half.

In this embodiment, the AlGaN/GaN HEMT of one kind selected from thefirst and second embodiments and the modification examples thereof isapplied to the PFC circuit 20. Consequently, the PFC circuit 30 havinghigh reliability is realized.

Fourth Embodiment

In this embodiment, a power supply device including the AlGaN/GaN HEMTof one kind selected from the first and second embodiments and themodification examples thereof will be disclosed.

FIG. 27 is a connection diagram illustrating a schematic structure ofthe power supply device according to the fourth embodiment.

The power supply device according to this embodiment includes ahigh-voltage primary-side circuit 31, a low-voltage secondary-sidecircuit 32, and a transformer 33 disposed between the primary-sidecircuit 31 and the secondary-side circuit 32.

The primary-side circuit 31 includes the PFC circuit 20 according to thethird embodiment and an inverter circuit, for example, a full bridgeinverter circuit 30 connected between the both terminals of thecapacitor 25 of the PFC circuit 20. The full bridge inverter circuit 30includes a plurality of (four here) switch elements 34 a, 34 b, 34 c, 34d.

The secondary-side circuit 32 includes a plurality of (three here)switch elements 35 a, 35 b, 35 c.

In this embodiment, the PFC circuit included in the primary-side circuit31 is the PFC circuit 20 according to the third embodiment, and theswitch elements 34 a, 34 b, 34 c, 34 d of the full bridge invertercircuit 30 are each the AlGaN/GaN HEMT of one kind selected from thefirst and second embodiments and the modification examples thereof. Onthe other hand, the switch elements 35 a, 35 b, 35 c of thesecondary-side circuit 32 are each a normal MIS FET using silicon.

In this embodiment, the PFC circuit 20 according to the third embodimentand the AlGaN/GaN HEMT of one kind selected from the first and secondembodiments and the modification examples thereof are applied to theprimary-side circuit 31 being the high-voltage circuit. Consequently, ahighly reliable, high-output power supply device is realized.

Fifth Embodiment

In this embodiment, a high-frequency amplifier including the AlGaN/GaNHEMT of one kind selected from the first and second embodiments and themodification examples thereof will be disclosed.

FIG. 28 is a connection diagram illustrating a schematic structure ofthe high-frequency amplifier according to the fifth embodiment.

The high-frequency amplifier according to this embodiment includes adigital predistortion circuit 41, mixers 42 a, 42 b, and a poweramplifier 43.

The digital predistortion circuit 41 compensates a nonlinear distortionof an input signal. The mixer 42 a mixes the input signal whosenonlinear distortion is compensated with an AC signal. The poweramplifier 43 amplifies the input signal mixed with the AC signal and hasthe AlGaN/GaN HEMT of one kind selected from the first and secondembodiments and the modification examples thereof. Incidentally, in FIG.28, for example, changeover of a switch enables the mixer 42 b to mix anoutput-side signal with an AC signal to send the resultant to thedigital predistortion circuit 41.

In this embodiment, the AlGaN/GaN HEMT of one kind selected from thefirst and second embodiments and the modification examples thereof isapplied to the high-frequency amplifier. Consequently, a high-frequencyamplifier high in reliability and high in withstand voltage is realized.

Other Embodiments

In the first and second embodiments and the modification examplesthereof, the AlGaN/GaN HEMTs are exemplified as the compoundsemiconductor device. The compound semiconductor device is applicable tothe following HEMTs in addition to the AlGaN/GaN HEMT.

Example 1 of Other Device

In this example, as the compound semiconductor device, an InAlN/GaN HEMTwill be disclosed.

InAlN and GaN are compound semiconductors whose lattice constants can bemade close to each other by their compositions. In this case, in theabove-described first and second embodiments and the modificationexamples thereof, the electron transit layer is formed of i-GaN, theintermediate layer is formed of AlN, the electron supply layer is formedof n-InAlN, and the first and third caps of the cap layer are formed ofn-GaN. Further, since almost no piezoelectric polarization occurs inthis case, two-dimensional electron gas is mainly generated byspontaneous polarization of InAlN.

Similarly to the above-described AlGaN/GaN HEMT, this example realizes ahighly reliable InAlN/GaN HEMT in which a protection film (firstinsulating film) is formed to have excellent insulating film quality butthe occurrence of an off-leakage current is surely inhibited, enabling areduction in a loss at the power-off time.

Example 2 of Other Device

In this example, as the compound semiconductor device, an InAlGaN/GaNHEMT will be disclosed.

GaN and InAlGaN are compound semiconductors whose lattice constants canbe adjusted by their compositions so that the lattice constant of thelatter becomes smaller than the lattice constant of the former. In thiscase, in the above-described first and second embodiments and themodification examples thereof, the electron transit layer is formed ofi-GaN, the intermediate layer is formed of i-InAlGaN, the electronsupply layer is formed of n-InAlGaN, and the first and third caps of thecap layer are formed of n-GaN.

Similarly to the above-described AlGaN/GaN HEMT, this example realizes ahighly reliable InAlGaN/GaN HEMT in which a protection film (firstinsulating film) is formed to have excellent insulating film quality butthe occurrence of an off-leakage current is surely inhibited, enabling areduction in a loss at the power-off time.

The above-described embodiments each realize a highly reliable compoundsemiconductor device in which a protection film is formed to haveexcellent insulating film quality but the occurrence of an off-leakagecurrent is surely inhibited, enabling a reduction in a loss at thepower-off time.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A compound semiconductor device comprising: anitride semiconductor region; an element isolation structure whichdemarcates an element region by an insulating material on the nitridesemiconductor region; a first insulating film which is formed on theelement region and is not formed on the element isolation structure; anda second insulating film which is formed on at least the insulatingmaterial of the element isolation structure and is higher in hydrogencontent than the first insulating film.
 2. The compound semiconductordevice according to claim 1, wherein the hydrogen content of the firstinsulating film is 1% or less, and the hydrogen content of the secondinsulating film is 1% or more.
 3. The compound semiconductor deviceaccording to claim 1, wherein the first insulating film and the secondinsulating film are each made of a material of aluminum oxide, hafniumoxide, aluminum oxynitride, or tantalum oxide or any combinationthereof.
 4. The compound semiconductor device according to claim 1,further comprising an electrode at least part of which is formed on thefirst insulating film in the element region.
 5. The compoundsemiconductor device according to claim 4, wherein the electrode isformed above the nitride semiconductor region in the element region, viathe first insulating film.
 6. The compound semiconductor deviceaccording to claim 4, wherein the electrode is in contact with thecompound semiconductor region in the element region via an openingformed in the first insulating film.
 7. A power supply devicecomprising: a transformer; and a high-voltage circuit and a low-voltagecircuit that sandwich the transformer, the high-voltage circuitincluding a transistor; the transistor including: a compoundsemiconductor region; an element isolation structure which demarcates anelement region by an insulating material on the compound semiconductorregion; a first insulating film which is formed on the element regionand is not formed on the element isolation structure; and a secondinsulating film which is formed on at least the insulating material ofthe element isolation structure and is higher in hydrogen content thanthe first insulating film.